In Patent Literature 1 (Japanese Patent Application Laid-open No. 2007-86720) is disclosed a polarized imaging apparatus that includes: a polarizer array divided in two or more types of polarizer areas each having a different transmission axis (i.e., a different polarization direction); a light receiving element array that independently receives the light that has passed through each polarizer area; and an image processing unit that processes polarization components received from the light receiving element array. In this polarized imaging apparatus, the polarizer array is configured either with polarizers made of photonic crystals or with wire grid polarizers.
In Patent Literature 2 (Japanese Patent Application Laid-open No. 2009-55624) is disclosed an image processing method in which an imaging apparatus including a color polarization obtaining unit, which has a color filter and a patterned polarizer arranged in a superposed manner on the front face thereof, is used so that color images and polarization information can be obtained in a concurrent manner. In this imaging apparatus, as the color filter, a Bayer color mosaic filter is used in which color filter units of different colors (RGB) are formed each corresponding to a set of four adjacent pixels of an imaging element. With the set of four adjacent pixels corresponding to each color filter unit, a patterned polarizer having a different principal polarization axis adheres closely. In this imaging apparatus, four pixels of an imaging element form a single set, and four types of polarization information can be obtained for each color of RGB.
In the imaging apparatus disclosed in Patent Literature 2 mentioned above, spectroscopic information (wavelength-band-specific information) and polarization information (polarization-direction-specific information) for each color of RGB can be obtained by capturing images only once. However, in Patent Literature 2, no concrete explanation is given regarding the method of manufacturing the color polarization obtaining unit, which has a color filter and a patterned polarizer arranged thereon in a superposed manner. Meanwhile, in recent years, pixel spacing in imaging elements (light receiving elements) has decreased to extremely narrow levels. Irrespective of that, in Patent Literature 2, no concrete explanation is given whatsoever regarding the method of forming the color filter units each having a different color (RGB) for a miniscule area of a set of four adjacent pixels of a light receiving element; or regarding the method of forming the patterned polarizer having a different principal polarization axis for each miniscule area such as a single pixel of a light receiving element; or regarding the method of arranging the color filter and the patterned polarizer in a superposed manner.
Meanwhile, in Patent Literature 1, there is an explanation in concrete terms of the method of manufacturing the polarizer array (polarization filter) that is divided in two or more types of polarizer areas each having a different transmission axis (i.e., a different polarization direction) for each miniscule area. Besides, regarding a color filter (spectral filter) in which the color filter units each having a different color for a miniscule area are formed, the microfabrication technology of recent years can be used to make such an arrangement feasible. However, a diligent research by the inventor(s) of the present invention found that following issues arise when a spectral filter and a polarization filter are arranged in a superposed manner.
In the case of arranging a spectral filter and a polarization filter in a superposed manner in a microscopic structure, a common manufacturing method is to form a laminate structure in which spectral filter layers and polarization filter layers are sequentially formed on a transparent filter substrate. Moreover, in order to obtain a polarization filter layer that is segmented into areas each having a different polarization direction for each miniscule area of a single pixel or a few pixels of a light receiving element; it is desirable to use a polarizer structure that is configured either with polarizers made of photonic crystals or has wire grid polarizers as described in Patent Literature 1 mentioned above and that is suitable for a microscopic structure. However, a polarization filter layer with such a polarizer structure happens to have an uneven top face. For that reason, if an attempt is made to form a spectral filter layer on that polarization filter layer, then the spectral filter layer also gets formed along the uneven top face of the polarization filter layer. That causes irregularity in the layer thickness of the spectral filter layer, thereby resulting in non-uniformity in the spectral performance of the spectral filter layer in the direction of light receiving element surfaces in the image sensor.
Furthermore, as far as a spectral filter layer is concerned on which filter areas are formed with each filter area having a different color (i.e., a different wavelength band) for each miniscule area; in connection to the fact that the filter area of each color needs to be formed from a different material, it is difficult to have identical layer thickness among the filter areas. For that reason, the top face of a spectral filter layer also becomes uneven. As a result, in the case of forming a polarization filter layer on a spectral filter layer, irregularity occurs in the layer thickness of the polarization filter layer. That causes non-uniformity in the polarization performance of the polarization filter layer in the direction of light receiving element surfaces in the image sensor.
The present invention has been made in view of the abovementioned issues, and it is an object of the present invention to provide an imaging device that, in a structure in which polarization filter layers, each segmented into areas on the basis of miniscule areas equivalent to a single pixel or a few pixels of a light receiving element, and spectral filter layers are arranged in a laminated manner, enables achieving curbing of irregularity in the layer thickness of those layers and enables fulfillment of the original functions of each layer; as well as to provide an object detecting apparatus including the imaging device, and to provide optical filters and a manufacturing method of the optical filters.